Method of applying highly heat resistant protective coatings to metallic surfaces



United States m METHOD OF APPLYING HIGHLY HEATRESIST- ANT PROTECTIVE COATINGS T METALLIC SURFACES Karl-Heinz Schmidt, Gelsenlrirchen, and Rudolf Brodt and Karl Lampatzer, Frankfurt am Main, Germany, assign ors, by mesne assignments, to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Aug. 25, 1960, Ser. No. 51,763

Claims priority, application Germany Aug. 28, 1959 9 Claims. (Cl. 1486.15) 1 The present invention relates to a novel method of applying highly heat resistant protective coatings to metallic surfaces.

This application discloses an improvement in and is a continuation-in-part of the invention disclosed in patent application Serial No. 738,957, filed June 2, 1958, now abandoned.

.' PRIOR ART A number of methods are known for applying protective coatings having a high heat resistance to metallic surfaces. Among these methods are those in which metallic oxides in the form of powder or of an aqueous suspension of oxides or hydroxides are applied to the surfaces and" the heat resistant protective coating is producedby a buming-in reaction. In this case there are various possibilities as to the form in which the reactions of the oxidic constituents with the metallic surface can progress. Thus it is possible for constituents of the metallic surface, for example, silicon or silicon dioxide, to' react chemically with the applied powder or the suspension or thesolution.

Such protective coatings may accordingly be of oxidic nature, but may also. have a glass-like character.

Thus it is known to apply a suspension of oxides or hydroxides of magnesium or calcium to thesurface of iron strip of low Si content, and to subject it to an an- Moreover, methods are known of applying-heat resistant phosphate coatings to metallic surfaces, if desired by the burning-in process. In this case, use has been made of solutions of phosphoric acid or of unstable phosphates, the non-phosphatic components of which, bound with the phosphoric acid, volatilize or decompose at the burning-in temperature. The phosphate coatings applied in this way consist of iron phosphate and with such solutionscoatings are obtained which, particularly in inert atmosphere, withstand temperatures of above 600 C. while i the known zinc phosphate coatings are no longer resist-' ant even at about 450 C. even in inert atmosphere. The type of coating is varied by the addition of cations such as alkaline-earth metals and especially calcium, magnesium, and aluminum which enter the coating and form heat resistant phosphates. The phosphate coatings contain alkaline-earth or aluminum phosphate. Such coatings are considerably more temperature resistant, namely up to about 1000 C. in an inert atmosphere. In oxidizing atmospheres, however, the limit of resistance of the coatings in the case of relatively long annealing treatment is about 700 0., since from this temperature onwards the 3,151,000 Patented Sept. 29, 1964 ice 2 oxygen diffuses through the coating and sets up oxidation of the basic material. g

It has also been proposed to add to the solutions to be burned-in, inert fillirig agents, for example, silicatesor temperature resistant oxides such as TiO Cr O ZrO Coatings obtained from such solutions by burning-in have a slight increase in temperature resistance in inert atmospheres. i

All these heat-resistant phosphate coatings applied by burning-in are accompanied by the disadvantagethat n1 though they are firmly combined with the metal they tend to rub off when subj ted to frictional stress, which is usually unavoidable in industrial concerns. Thusbn such layers, for example, it is possible to remove the coating just with the fingernail.

In order to increase the adhesion, a method has already been proposed in which useis made of a two-stage buming-in process. The phosphate coating burned-in in the first stage is destroyed by an annealing treatment, for example, by annealing for several hours in an atmosphere of hydrogen such as is employed for the grain orientation of transformer laminations. A phosphate coating is then again applied in a second burning-in process to this surface, on which there still may be residual components of the destroyed phosphate coating. If solutions containing alkaline-earth phosphates are used with this method, a highly heat-resistant phosphate coating is obtained which is certainly very much better combined with the metallic surface, but even a coating applied in this way does not obstruct oxygen diffusion at temperatures from 700 C. upwards. Nor can the resistance of the coating to abrasion be substantially increased by this method.

If inert filling agents, for example, mica, such as is known especially for electrical insulating coatings for the purpose of increasing the insulating action, are added to such solutions for forming the protective coatings, the process is rendered difficult by the readily depositing filler. Since the solution to be used is usually distributed uniformly on the metallic surfaces with the aid of rollers, the deposition of the filler in the grooves of the rollers is disturbing. Furthermore, these inert fillers tends to be deposited on the transport rollers in the furnace during the'burning-in reaction. These deposits can be removed only with difficulty and have an adverse etfec't on the surface quality of the treated material. In the case of electrical laminations the space factor is considerably reduced in consequence.

U.S. APPLICATION S.N. 738,957, FILED JUNE 2, 1958 US. application S.N. 738,957, filed June 2, 1958, relates to a process for applying highly heat resistant protective layers on the surfaces of metallic objects, such as, sheets or strips of metal made, in particular, of iron, iron alloys or steel, in which the surface of the metal object is caused to react with one or more metal oxides and/or hydroxides that do not originate in nor are formed from the metal object, with the formation of a non-metallic base coating on the'metal object and this non-metallic base coating, in turn, is treated with a burning-in phosphatising process. This process is of particular significance for the productionof electrically insulated layers or coatings on electrically used sheets and strips of metal, particularly for transformer plates and transformer strips having magnetic preferential direction.

By a burning-in phosphatising process it is meant, as it is known to the art, a phosphatising process wherein a film of solvent containing phosphorous compounds is applied to a metal surface and these compoundsare then burned into the metal surface to form a phosphate coating thereon. The formation of the phosphate coating takes place during the burning-in operation.

. of producing certain For the purpose of applying the base coating, the metallic oxides or hydroxides can be applied to and caused to react with the base material in the form of oxides or hydroxides, in solution or in suspension. It is also possible, however, to form on the surface of the object the oxides to be caused to react. In this case, it is possible to apply to the object and burn-in a solution or suspension of one or more compounds of the metal or metals to be caused to react as oxides and/or hydroxides, in which case the oxides or hydroxides which react with the surface form in the layer applied. Drying may be carried out before the burning-in process. If desired, the formation of oxides and/or hydroxides or the reaction with the surface can be promoted by special adjustment of the -'atmosphere.

Particularly suitable as metallic hydroxides or oxides to be made to react with the surface are those of magnesium, calcium, chromium, iron, aluminum, manganese, alone or in combinations.

Other components may jointly participate in the reaction resulting in the formation of the non-metallic base coating. For example, the formation of the base coating may be carried out in the presence of silicon in free or combined form. In this case it may be sufficient for the base material to be siliconcontaining and the silicon of the base material to take part in the reaction. However, it is also possible to introduce or make additionally available silicon in suitable form with the solutionor suspension from which the base coating is formed.

- The oxides or hydroxides, especially when using carbonates may first be formed on the surface to be protected, these carbonates being applied in the form of solutions or suspensions and being decomposed by heating. It is also possible to use, instead of carbonates, other salts unstable at feasible burning-in temperatures, for example, sulphates, acetates, oxalates or nitrates. i

The presence of carbon has also proved favorable in the formation of the base coating. For example, an organic compound which decomposes at the burning-in temperature may be added to the solutions containing the oxide or hydroxide components or the compound from which the oxide or hydroxide forms on the surface.

To form the base coating, the workpiece on which the oxides and/ or hydroxides or the compounds from which these are formed are applied, is preferably subjected to an annealing, which is preferably carried out at temperatures of 800 to 1350 C. Annealing of the applied oxide layer or carbonate or hydroxide layer is effected at tempertures at which the progress of the reaction with the metal surface is ensured. It may be advantageous so to control the annealing treatment thatrather insoluble oxides are embedded in the coating, for example, MgFe O FeCr O and similar known highly temperature resistant oxides. This can be effected for example by applying and burning-in together with MgO iron oxides or chrome oxide, or else by adjusting the annealing conditions in such manner that iron oxides can form from the base metal. This annealing may advantageously be combined with a thermal treatment of the base material, for example, with a recrystallization annealing, for the purpose physical properties, for example, development of a magnetic preferential direction in transformer lamin'ations.

In order to obtain firmly combined coatings which do not chip off even under bending stress, it is necessary to make the base coating as thin as possible. The substances necessary for the reaction are preferably applied in a coating so thin that the base coating formed by reaction with the surface of the workpiece amounts to about 1 to 3 microns. This can be achieved, for example, by applying the oxide suspension in a thin uniform distribution by means of squeeze rollers. The amount of material to be used can be varied and uniformly distributed, by varying the squeeze pressure and/or the profiling of the squeeze rollers and/or the consistence or concentration of the suspension. The thickness of the suspension coating applied does not always determine the thickness of the base coating to be formed by reaction, since the reaction with the surface can also be concluded earlier before all the metallic oxide or hydroxide present has reacted. The excess not participating in the reaction can easily be removed mechanically after the reaction is finished.

This pre-treatment stage, in which a base coating is applied, is then followed by the burning-in phosphatising. It is surprising that phosphatisation occurs with a material covered with such a base coating. Furthermore it has even been found that it results in a particularly firmly anchored coating. This phosphatisation can be carried out by one of the aforedescribed known processes or other known methods. Both the solutions consisting of only unstable phosphates, that is, phosphates the radicals of which, combined with phosphoric acid, volatilize or burn at higher temperatures, so that they are not taken up into the burned-in layer, for example, ammonium phosphate, urea phosphate and guanidinium phosphate, and the solutions containing the alkaline-earth, magnesium and/ or aluminum phosphate, can be burned on to the surfaces provided with the base coating and yield firmly combined phosphate coatings. Particularly suitable are the solutions which in addition to alkaline-earth metal phosphate, especially calcium phosphate, also contain at least one unstable phosphate, for example, ammonium phosphate. Small proportions of free phosphoric acid in these solutions can advantageously assist the coating formation reaction.

The treatment with the phosphatising solution, of the material provided with the base coating, is preferably effected with uniform distribution of the solutions on the surface, followed by burning-in at temperatures from 200 to 800 0., preferably at 500 to 700 C. Depending upon the burning-in temperature, burning-in times between 15 seconds and 2 minutes are sufiicient in this case.

In carrying out the hereindescribed method it is per se also possible to incorporate in the phosphatising solution inert fillers, especially highly heat resistant inert substances or compounds resulting in the same, for example, silicates, Cr O TiO and ZrO In detail, such additions may result in advantageous properties in the coatings for special applications, for example, a particularly good effect of protection against-adhesion in the case of annealing processes, or to obtain highly reflective coatings, such as are desired, for example, in combustion chambers.

In applying insulating coatings, for example, to electrical laminations, an adequate insulation effect was not hitherto obtained without the incorporation of mica. The introduction of mica into the insulation coating caused difiiculties, however, and in particular made it difficult to carry out the process, since owing to the deposition of mica in the bath solution mica also clogs the grooves of the squeeze rollers or is deposited on the transport rollers in the burning-in furnace. It is therefore a special advantage of the hereindescribed method that even without the incorporation of mica it enables coatings with willcient insulation effect to be applied, so that it is not necessary to suspend mica in the treatment solution. The coatings applied according to the invention without the use of mica are even superior, in their insulation effect and quality, to those which were hitherto applied using mica, comparison being made of the insulation properties of identical coating thicknesses. If, for any reasons, it should be desired to incorporate mica in the hereindescribed method as well, this is quite possible. The space factor, in the case'of electrical laminations, for example, is reduced in consequence, however, so that it is more favorable to work without mica for this reason also.

The thickness of the phosphatecoating to be applied and the type of coating depend on the requirements made of the coating. When the coatings are used for electricalinsulation purposes, the important factor is to obtain a high space factor with good insulation etfect.- For this purpose, coating thicknesses of about 1 to 3 microns on the surface of the workpiece are suflicient. This applies especially to the application of insulation coatings on grain-oriented transformer la'minations.

ln carrying out the hereindescribed method the appearance of the final coating is greatly dependent on the nature and thickness of application of the base coating. In this connection it is surprising that the burning-in of the phosphatising solution on the base coating generally does not result in any appreciable increase of the thicknes of the coating beyond that of the base coating.

The combination of the two burning-in reactions does not usually result in an ordinary superimposition of two different layers but, as has been found, a penetration of the two types of coating, with or without an additional chemical reaction taking place. Anchoring of the coating is thereby improved and an impermeable smooth protective coating formed. This is advantageous especially for the application of these protective coatings to electrical insulation, for example to electrical laminations, since the smooth surface and the comparatively small thickness of the coating result in excellent space factors with good insulating effect.

Each of the two treatments individually results in porecontaining coatings. The combination according to the invention of the two methods with one another, however, results in a very dense closed coating. The insulation effect is thus extremely high even with very small thickness of the coating. A high corrosion resistant action is secured in the same way by this process in consequence of the protective coatings.

In the known burning-in phosphatising methods use may be made of wetting agents. These are also permissible in the combined method according to this process, but are not necessary or else required only to a much smaller extent. A substantial advantage of the method according to the invention in fact is that the base coating applied has very favorable wetting properties, so that even if the phosphatising solution, does not contain wetting agents, this solution completely wets the base coating. (This is probably due to the oxidic character of the base coating.) The equalization of the liquid film of -the phosphatising solution on the base coating is thus considerably facilitated.

Particularly advantageous technically in the coatings applied according to this proces is the extremely high resistance to abrasion, so that no damage occurs during further conversion, for example, stamping and cutting, of

the treated sheets.

THE PRESENT INVENTION In conducting the process described in US. application S.N. 738,957, filed June 2, 1958, the appearance and utility of the final, combined protective layer are greatly dependent upon the type and thickness of the initially applied non-metallic base coating. It is unexpected in this regard that the burning-in of the phosphate solution into this base coating generally does not lead to any essential increase above the thickness of the base coating. This, of course, is explained by the unexpected fact that the combination of the two burningin operations, that is, one for the base coating and one for the phosphatising operation, does not involve the customary superpositioning of two different layers one on top of the other, but results rather in a mutual permeation of the two layers. As a result of this mutual permeation of the two layers it was found that not only does the base coating not seal off access to the surface of the metal object with a dense impenetrable coating but that by reason of the porous nature of the base coating a reaction is possible between the burned-in phosphate solution and the surface of the metal object through the porous base coating.

It has also been shown, that for the improved utility of the combined layer, in particular if the combined layer is to be used as a heat resistant coating for the electrical insulation of electrical plates and strips, that a definite minimum thickness is necessary for the base, non-metallic coating and that base coatings of this type having an adequate minimum thickness must also be permeable to accommodate the burned-in phosphate materials in the second burning-in reaction.

It has now been found according to the present invention that the formation of such permeable base coatings is facilitated and favorably influenced if care is taken during the formation of these base coatings that the reaction between the oxide and/or hydroxides, which are not formed from, nor originate in, the metal object, and the surface of the metal object takes place with the aid of reactable oxygen, which is present at that portion of the metal objects surface which is to be coated. By reactable oxygen it is meant, in the present invention, oxygen which is capable of rendering an oxidizing effect to the reaction area, that is, on the surface of the metal object to be coated during the formation of the base coating. In maintaining suflicient quantities of reactable oxygen in the reaction area due consideration must be given to the possible loss of said reactable oxygen from the reaction zone by diffusion. The introduction of oxygen into an atmosphere, in which its oxidizing effect is destroyed, however, is not what is meant here, since the present invention calls for the use of reactable oxygen.

Since the production of electric sheet materials involves the use of several heat treatment steps, the first heat treatment which is-required for the formation of the base coating according to the invention can be combined with one of the heating steps used in the processing of the electric sheet materials. For this purpose it would be preferable to use the final heating step of the customary electric sheet material heating process. In the production of electric strip material, particularly those in which a magnetic preferential direction is desired, it has been customary to process these strip materials in a reducing atmosphere or in a vacuum. It has been determined, however, that a suitable base coat formation could only be obtained if the atmosphere in the immediate reaction zone itself, that is, on the surface of the strip to be coated, during the chemical reaction involved in the heat treatment to produce the base coating according to the invention is not a reducing atmosphere but is slightly oxidizing. A base coating is not obtained in a reducing atmosphere during the customary process of annealing electroplates and strips, since the measures necessary for the maintenance of the reducing atmosphere there, also have the effect of removing residual moisture values by the time temperatures are attained at which a possible reaction of the magnesium oxide which is applied in a moist condition to the surface of the metal strips in the prior art processes to prevent the adhesion of the coiled metal strips together, with the surface of the metal strip could take place. This is not achieved in the normal prior art procedures for annealing electrical strip mate rial as the residual moisture values escape from the coiled metal strips, which have had a moist film of MgO applied thereto to prevent the strips from sticking together during the subsequent annealing treatment, into the annealing chamber at about 320 to 350 C. and are removed from there by the means used to maintain the reducing atmosphere of the annealing chamber. By the time temperatures of 700 to 850 C. are reached, in the prior art processes, at which the reaction between magnesium oxide and the surface of the metallic object could take place in a reaction time of 10 to 4 hours, or temperatures of 1000 to 1100 C. at which a reaction time of 1% to /2 hour would be sufficient, no residual moisture is present on the surface of the metal object during the annealing process in the reducing atmosphere so that even though there is MgO present on the metal surface no coating on the metal surface occurs.

The reactable oxygen of the present invention can be supplied to the reaction zone in a free or combined form. In fact, the residual moisture, that is, chemically uncombined water, which originates from the solution or suspension of the metal oxide and/or hydroxides, is capable of forming a slightly oxidizing atmosphere on the metal surfaces on which it is located and can be used to facilitate the formation of the base coating, provided, .of course, that measures are taken to ensure its presence in the reaction zone while the temperature is suitably high for carrying out the formation of this porous, nonmetallic base coating. The reactable oxygen, can also be supplied in the form of hydroxides, carbonates or other oxygen containing materials present in the layer of chemicals to be used to form the base coating which will decompose during the annealing or burning-in operations and yieldthe necessary reactable oxygen, for example, as water, CO or directly as molecular or atomic oxygen. For example, magnesium carbonate decomposes at 400 C. into magnesium oxide and carbon dioxide, the latter of which is a source of reactable oxygen within the meaning of the present invention.

All these forms of reactable oxygen are suitable for furthering the course of the reaction between the surface of the metal object and the metal oxides and/or hydroxides to be used to form the non-metallic base coating.

It was also discovered that the formation of the nonmetallic base coating can be favorably influenced by regulating the amount of reactable oxygen present, during the course of the reaction, on the surface area of the metal object on which the formation of the base coating is to take place. It is possible, for example, to so direct the course of the heating operation after applying the oxide and/or hydroxide solution or suspension to the metal surface to ensure that residual moisture values are still present in' the reaction zone at those temperatures at which the reaction occurs between the metal surface or its component parts and the applied oxides and/or hydroxide which do not originate in the metal surface.

Reactable oxygen, which is to be made available solely in the form of residual moisture in a given case, can only.

be retained on the reactive surface area with a great deal of difficulty due to the high annealing temperatures used. However, in order to ensure that suthcient quantities of reactable oxygen are'present in the form of moisture at the temperatures necessary for the successful operation of the process it is possible to provide for additional, chemically combined water which becomes available at the reaction temperature. By regulating the quantities of hydroxides and choosing the particular one to use, it is possible to have reactable oxygen present during the heat treatment even at the required temperatures on the boundary surface between the metal object and the layer of oxide and/ or hydroxide applied to it for the base coating reaction since different hydroxides will decompose and yield free water at different temperatures. If care is to be taken that reactable oxygen is to be present in the reaction zone at the temperatures at which the particular hydroxide have already decomposed then one can include oxygen containing compounds in the oxide or hydroxide coating which decompose at these higher temperatures, such as, carbonates or oxygen rich oxides, which are transformed when heated into a lower valent form of the metal. Among others, this class of compounds would also include such compounds as chromates, molybdates and E210- There is, however, not only the possibility in working with this reactable oxygen in the present invention of ensuring that renewed quantities of the reactable oxygen are made available and distributed over the whole range of the reaction temperature ranges but there is also the possibility of retarding the diffusion of the reactable oxygen from the reaction area. In this regard it has proven to be particularly helpful to embed the goods to be annealed in the form of stacked sheets or coiled strips,

on whose surface the oxide and/or hydroxides to be used in thereaction have already been applied and dried thereon, in a mass of powdery material which will not interfere with the subsequent formation of the base coating. By means of these embedding powdery materials, the escape of the reactable oxygen or its carrier from the area in which the base coating forming reaction is to take place, is prevented and the reaction is thereby promoted.

By suitable choice of the inert powdery material used as well as the thickness of the embedding layer it is possible to obtain the most favorable conditions, since the reactable oxygen preferably should not be retained too long on the surface of the metal object but should be removed or allowed to diffuse away at the termination of the formation of the base coating since residual oxygen values can have a detrimental effect on the base metal. This detrimental effect, for example, includes the formation of undesirable characteristicss in the insulation coatings on the electroplate materials which debase the latters magnetic and mechanical properties.

The embodiment of the process of the present invention, in which the period of time that the reactable oxygen or its carrier is effectively maintained in contact with the reaction area is regulated by the above described embedding technique, is particularly adapted to control the reaction involved in the formation of the non-metallic base coating and, for example, to retard the diffusion of reactable oxygen from the reaction zone only so long that when the desired thickness of the base coating layer is attained the reactable oxygen will either be all used up or will have diffused away from the reaction zone. Only in this Way is it possible to prevent the occurrence of disturbances in the treated materials from arising during the subsequent annealing treatments, particularly when they are conducted at the higher temperatures.

The use of a slightly oxidizing atmosphere in the annealing chamber, as is used occasionally by the prior art in the treatment of electrosheet materials, is not sufficient in itself with stacked plate or coiled sheet goods in order to aid in the formation of the base coating layer, since the reactable oxygen cannot penetrate into the reaction zones of the densely packed materials in sufficient quantities to be of much help. a

When selecting the powdery embedding materials to be used care should be taken that those chosen do not undergo deleterious side reactions with the base metal. Of particular utility in this regard are powdery oxide materials, such as are used for the formation of the base coating with the metal surface. Oxides that are suitable for this purpose include the high melting oxides of magnesium, aluminum, calcium, titanium and silicon used alone or in combination with one another.

The retardation of the escape of reactable oxygen from the reaction zone, which is effected by these embedding masses, can be regulated by various combinations of the thickness of the layer of embedding material used as well as the packing density and granule size of the embedding materials. After the reaction involving the formation of the base coating has come to an end then the residual reactable oxygen values can be allowed to diffuse into the other parts of the annealing chamber, if such excess quantities are still present.

The following examples are merely illustrative of the present invention and are not intended as a limitation upon the scope thereof.

Example 1 An aqueous slurry of a powder containing MgO and 10% Mg(OH) was applied to both sides of a strip of an iron-silicon alloy. After removal of some of the water to provide an adhering film of MgO and Mg(OH) this strip was placed in coiled state in an annealing box open at the top and embedded therein in fine powdered, carbonate free, MgO. The depth of the magnesia, which had been slightly compacted, over the upper edge of the coiled strip amounted to about 150 mm. and the distance from the annealing box to the outer coil of the strip was about 80 mm.

The annealing box, with its contents, was then placed in an annealing furnace.

The annealing was conducted for several hours at 1050 C. in a protective gas atmosphere. When the electrostrip materials are to be used in such a way as to require a grain orientation therein, then the annealing operation is conducted, as is known to the art, in a nitrogen atmosphere using the heating times and temperatures necessary to form a magnetic preferential direction of the grain structure. If necessary, the annealing process can be carried out in a vacuum or in a dry hydrogen atmosphere so that the base coating layer that is formed is not impaired. After the strips were cooled down, the excess, unreacted oxide materials can be mechanically removed, for example, by brushing. The base coating layer thereby obtained had a uniform thickness of 2,u.. Thereafter, an aqueous phosphating solution having the composition:

150 g./l. primary ammonium phosphate 140 g./l. monocalcium phosphate, and

27 g./l. free P was applied to the base coated metal strip at room temperature. This solution completely moistened the surface of the metal strip. A uniform distribution of the solution over the metal strips surface was attained with the aid of grooved rubber rollers and the solution coated strips were then placed in an annealing oven, at 680 C. for the phosphatising burning-in process. The burningin time amounted to 60 seconds. A smooth, uniform insulation coating was thereby obtained which had a thickness of 3,11..

In another test, using metal strips, base coated as above, an aqueous solution having the. following composition 200 g./l. primary calcium 37 g./l. free P 0 was uniformly applied to the metal strips and thereafter burned into the base coating by heating the phosphate solution coated strips at 550 C. The phosphatising burning-in time was 2 minutes. A single, compact, interpermeated, smooth protective layer having a particularly fine crystalline dense structure was thereby obtained.

phosphate and Example 2 A strip of an iron-silicon alloy composition was coated with an aqueous slurry of a powder consisting of 30% A1 0 52% MgO; and 18% MgCO and was thereafter heated to below 400 C. (which is the decomposition temperature of the carbonate) until all the water, including the chemically combined water, had been removed.

Then the strip was coiled up and placed in an annealing box, the diameter of which was 100 mm. greater than that of the coiled strip. The annealing box and contents were placed in an annealing oven. The unoccupied space in the annealing box was filled with lightly compressed, pure alumina. The depth of the alumina over the top of the coiled strip was about 100 mm. The annealing was conducted for several hours at 950 C.

The phosphatising burning-in operations were conducted as in Example 1.

Example 3 Electrosheet materials, consisting of an iron-silicon alloy, were coated with an aqueous suspension of a powder consisting of 92% MgO 6% Mg(OH) 1% CrO and 1% B210;

10 and then dried at 300 C. The plates were then stacked in an oven and subjected to an annealing treatment for several hours at 900 C. in an inert atmosphere. After the plates were cool the excess quantities of the applied materials which had not reacted with the metal surface were brushed off and the plates were then treated with a phosphatising burning-in process as described in Example I. By this procedure, an insulated coating layer was obtained of outstanding dielectric strength with a thickness of 3.5 .1.

Example 4 A strip of iron-silicon alloy material was coated with an aqueous suspension of a powder consisting of 30% A1 0 52% MgO, and 18% CaCO and the coated strip was heated at 400 C. until all the water, including the chemically combined water, had been removed.

The strip was coiled and placed in an annealing box, the diameter of which was about mm. larger than that of the coiled strip. The annealing box and contents were then placed in an annealing oven. The unoccupied space in the annealing box was filled with slightly compressed, pure alumina. The height of the alumina over the top edge of the coiled strip was about 100 mm. The annealing treatment was conducted for several hours at 950 C. The calcium carbonate began decomposing at about 800 C. and the CO set free provided available reactable oxygen in the reaction zone.

The phosphatising operations were conducted as in Example I.

We claim:

1. A process of applying highly heat resistant protective coatings to ferrous metal objects comprising providing the surface of the said metal objects with a layer of at least one oxygen containing metal compound supplied from an external source and selected from the group consisting of metallic oxides, metallic hydroxides and metal compounds which produce such oxides on heating. heating said metal objects provided with such layer to a temperature between about 800 and 1350" C. while maintaining an oxidizing condition directly at such surface of the metal objects to cause said oxygen containing metal compound in such layer to react with the surface of the said metal objects to form a porous non-metallic base coating, applying a film of an aqueous solution containing at least one phosphating compound selected from the group consisting of phosphoric acid, aluminum phosphates, alkaline earth metal phosphates and heat decomposable phosphates to said base coating and burning-in said solution containing the phosphatizing compound at a temperature between 200 and 800 C. whereby said phosphatizing compound forms a combined interpermeated protective coating with said base coating.

2. A process as in claim 1 in which said oxidizing condition is produced in situ in the layer containing said oxygen containing metal compound.

3. A process as in claim 2 in which the source of said oxidizing condition is at least one compound selected from the group consisting of water and compounds which produce an oxidizing condition when decomposed by heat admixed with said oxygen containing metal compound.

4. A process as in claim 3 in which the decomposition temperature range of said heat decomposable compounds approximates the operating temperature range of the reaction between said oxygen containing metal compound and the surface of said metal object.

5. A process as in claim 1 in which said burning-in is conducted at a temperature of 500 to 700 C.

6. A process as in claim 1 in which the thickness of the base coating obtained is substantially determined by the duration of the presence of said oxidizing condition 1 1 during the reaction between said oxygen containing metal compound and the surface of said metal object.

7. A process as in claim 1 in which said articles provided with the layer of said oxygen containing metal compound are embedded in a powder of a high melting oxide selected from the group consisting of oxides of magnesiutn, aluminum, calcium, titanium and silicon.

8. A process of coating transformer and dynamo ferrous sheet and strip materials containing silicon comprising uniformly providing the surface of said materials with a layer of an aqueous system containing at least one oxygen containing compound supplied from an external source selected from the group consisting of metal oxides, metal hydroxides and metal compounds which produce such oxides on heating, predrying said materials provided with such layer, arranging said materials in a tightly packed fashion, embedding said arranged materials in magnesium oxide powder, heating said embedded materials to a temperature of 800 to 1350 C. while maintaining an oxidizing condition directly at such surface of such materials to cause said oxygen containing compound to react with the surface of said materials in the presence of reactable oxygen with the formation of a porous, non-metallic base coating, applying a film of an aqueous solution containing at least one phosphating compound selected from the group consisting of phosphoric acid, aluminum phosphates, alkaline earth metal phosphates and heat decomposable phosphates to said base coating and burning-in said solution containing the phosphatizing compound at a temperature between 200 and 800 C. whereby said phosphatizing compound forms a combined interpermeated protective coating with said base coating.

9. A process as in claim 8 in which said oxygen containing compound is selected from the group consisting of magnesium oxide and magnesium hydroxide.

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1. A PROCESS OF APPLYING HIGHLY RESISTANT PROTECTIVE COATINGS TO FERROUS METAL OBJECTS COMPRISING PROVIDING THE SURFACE OF SAID METAL OBJECTS WITH A LAYER OF AT LEAST ONE OXYGEN CONTAINING METAL COMPOUND SUPPLIED FROM AN EXTERNAL SOURCE AND SELECTED FROM THE GROUP CONSISTING OF METALLIC OXIDES, METALLIC HYDROXIDES AND METAL COMPOUNDS WHICH PRODUCE SUCH OXIDES ON HEATING, HEATING SAID METAL OBJECTS PROVIDED WITH SUCH LAYER TO A TEMPERATURE BETWEEN ABOUT 800 AND 1350*C. WHILE MAINTAINING AN OXIDIZING CONDITION DIRECTLY AT SUCH SURFACE OF THE METAL OBJECTS TO CAUSE SAID OXYGEN CONTAINING METAL COMPOUND IN SUCH LAYER TO REACT WITH THE SURFACE OF THE SAID METAL OBJECTS TO FORM A POROUS NON-METALLIC BASE COATING, APPLYING A FILM OF AN AQUEOUS SOLUTION CONTAINING AT LEAST ONE PHOSPHATING COMPOUND SELECTED FROM THE GROUP CONSISITNG OF PHOSPHORIC ACID, ALUMINUM PHOSPHATES, ALKALINE EARTH METAL PHOSPHATES AND HEAT DECOMPOSTABLE PHOSPHATES TO SAID BASE COATING AND BURNING-IN SAID SOLUTION CONTAINING THE PHOSPHATIZING COMPOUND AT A TEMPERATURE BETWEEN 200* TO 800*C WHEREBY SAID PHOSPHATIZING COMPOUND FORMS A COMBINED INTERMEATED PROTECTIVE COATING WITH SAID BASE COATING., 